A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The lithographic apparatus may be of a type wherein an optical projection system is used to project a pattern imparted on a radiation beam by a patterning device, for instance a mask, onto a target portion of a substrate, as will be explained later. An optical projection lithographic apparatus may furthermore be of a type wherein at least a portion of the substrate may be covered by a liquid having a relatively high refractive index, e.g., water, so as to fill a space between the projection system and the substrate. An immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of projection systems. The term “immersion” as used herein does not mean that a structure, such as a substrate, must be submerged in liquid, but rather only means that liquid is located between the projection system and the substrate during exposure.
Furthermore, the lithographic apparatus may be of a type having two (dual stage) or more substrate tables supporting a plurality of substrates (and/or two or more mask tables). In such “multiple stage” machines the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables (i.e., in a preparatory stage or measurement stage), while one or more other tables are being used for exposure of one or more substrates (i.e., in an exposure stage).
An example of a preparatory step carried out in the measurement stage of an optical projection lithographic apparatus is the measurement of the distance between the optical projection system and the substrate to be exposed. The best resolution of an projection image is made when the substrate is in focus of the optical projection system. In order to obtain a good focus, the substrate should be positioned at the focal point of the optical system. In order to maintain the substrate surface within the focus of the projection system, the distance of the substrate from the optical system (especially the projection lens) should be determined accurately. To this end a lithographic apparatus may comprise a sensor for measuring the height of the surface of the substrate.
Currently a sensor for measuring the height of the substrate is used as a tool to measure the local height and tilt at a large number of measurement points on the surface of the substrate. In a typical dual stage lithographic apparatus, for instance the ASML Twinscan, the height information that is gathered in the measurement stage by the sensor (eg. the level sensor), is forwarded to the exposure stage and used during exposure of the substrate. In other types of lithographic apparatus the sensor, sometimes called the “focus sensor”, is configured to perform the substrate height measurements on the fly, that is during exposure of the substrate.
Although the height of the surface of the substrate is measured, the sensor fails to detect defect patterns present on the substrate. Defects typically occur at the substrate edges due to processing problems. Earlier in dry machines this was less of a problem as during exposure the substrate normally did not come into physical contact with the lithographic apparatus. In immersion type lithographic apparatus, because of the contact of the substrate with immersion water, there is a risk that defects might spread to other portions of the substrate, which may lead to further damage to the substrate and contamination of the lithographic apparatus as the process layers are peeled off from the substrate and flow with immersion liquid to various parts of the apparatus.